1. Field of the Invention
The present invention relates to a rotational speed detector for detecting a rotational speed such as a wheel speed of an automobile and to a method of manufacturing the detector.
2. Description of Related Art
An example of a conventional wheel speed detector which is installed on a vehicle to face a rotor rotating together with a vehicle wheel is disclosed in JP-A-11-14644. In this detector, a sensor element is disposed in a depressed portion of a sensor housing, and the sensor element and associated circuits are covered with a resin material to protect them against water. The sensor element covered with the resin is positioned to face the rotor. A sufficient air gap between the detector and the rotor is required to absorb a dimensional dispersion of those components and vibration of a driving axle due to driving on a rough road or rolling of a vehicle body. On the other hand, it is required to make a distance between the sensor element and the rotor sufficiently small to obtain sensor signals at a high level.
Because a resin layer covering the sensor element is interposed between the rotor and the sensor element in the conventional detector described above, it is necessary to make the resin layer thin to satisfy both requirements. However, it is difficult to make the resin layer thin because a certain thickness of the resin layer is necessary to provide a good sealing against water when the detector is used under severe environmental conditions.